The present invention generally relates to electrical discharge machining and, more particularly, to a hybrid servomechanism for micro-electrical discharge machining.
Electrical discharge machining (EDM), or spark erosion, is a method of machining conductive materials by applying a series of electrical sparks in the presence of a dielectric. It was serendipitously discovered by B. R. Lazarenko and N. I. Lazarenko in 1943 in the process of trying to remove a stuck drill bit from a hole by means of pulsed electrical discharges. As shown schematically in FIG. 1, a spark discharge is produced by the controlled application of DC voltage pulses between two electrodes, namely, the work-piece 10 and the tool (electrode 12), which are separated by a distance of approximately 0.01 mm to 0.50 mm (spark-gap). A dielectric fluid 14 is present in the spark-gap. Upon pulsed application of a high voltage, the dielectric 14 in the gap is partially ionized, thus causing a spark discharge between the tool 12 and the work-piece 10.
Each discharge produces enough heat to melt or vaporize a small quantity of the work-piece 10 material, and this material is ejected at the end of the discharge, creating a tiny pit or crater that is left behind on the surface of the work-piece 10. This is the mechanism of material removal. Even though in EDM tool wear is high and the machining rate is much smaller than in turning, milling or grinding, it has still found a wide range of applications. The facts favoring EDM over conventional machining processing in some applications are its ability to: (1) remove machine materials of high hardness, high tensile strength and poor machineability; (2) machine complex or irregular shapes and intricate cavities; and (3) fabricate parts that are too thin and fragile to withstand the forces produced in conventional machining. Furthermore, the manufactured component is free of burrs. The largest application of EDM is in the machining of dies and molds, either before or after hardening; machining of carbides, tungsten and more recently conductive ceramics such as titanium di-boride, boron carbide and silicon carbide composites. Die-sinking and wire-cut EDM are the commonly used configurations for these applications.
A more recent EDM process that was developed in the late 1960""s is micro-hole drilling, henceforth referred to as a micro-EDM process. An important application of this process is in the drilling of the small diameter (xcx9c150xcexc) orifice holes of fuel injector nozzles in diesel engines. Holes of diameter xcx9c100 xcexcm to 250 xcexcm and with an aspect ratio greater than 5 (aspect ratio being the ratio of the depth of the hole to its diameter) are very expensive to drill by conventional means. Frequent tool re-sharpening, excessive drill breakage, the poor ability of hard alloys to withstand machining and formation of entry or exit burrs with mechanical drills make conventional drilling almost impractical as a production process for producing such micro-holes. But with micro-EDM, the machineability is more a function of the melting point rather than the hardness of the work material, and it is inherently a burr-free process. In conventional drilling, the hole diameter is primarily determined by the diameter of the drill and the operator has little control over the size of the resulting hole. But by the suitable selection of process parameters, it is possible to control, within bounds, the amount over-cut in EDM. Hence, for a given diameter of the tool electrode 12, the operator can control and adjust the diameter of the hole. The dimensional accuracy of the holes (i.e., their size and taper) produced by micro-EDM is usually superior to that produced by other unconventional processes such as electro-chemical machining (ECM) and laser machining. Hence, micro-EDM has become an established production process for the drilling of small holes.
Much effort has gone into understanding the physics of the EDM process and to relating the instantaneous gap conditions to the process performance. In this process, the machining is carried out by a series of electrical discharges which are applied between the tool 12 and work surfaces 10 in the presence of a liquid dielectric medium 14. A relaxation type, or a pulse generator type, of power supply provides a DC voltage of 100 to 200 volts between the tool electrode 12 (usually the cathode) and the work piece 10 electrode (usually the anode). The tool electrode 12 for hole drilling is in the form of a thin circular wire which is guided through closely matched ceramic guides. Tungsten or a tungsten-copper alloy is commonly used as the tool electrode 12 material because of its low rate of wear. The dielectric 14 is usually de-ionized water, which is drip-fed into the gap between the tool 12 and the work-piece 10 surfaces. At a critical value of the applied voltage, the dielectric 14 breaks down, causing an electrical discharge to occur between the tool 12 and the work surface 10. During every such discharge a small volume of material is removed from the work-piece 10 surface as a consequence of localized melting and ejection of the molten material. The crater produced by the localized melting is usually small, typically a few micrometers in width. The cumulative effect of a succession of such discharges spread over the entire work-piece 10 surface leads to its erosion, or machining to a shape which is approximately complementary to that of the tool 12.
As machining occurs, a servo system 16 advances the wire (tool) 12 in order to maintain a preset gap of about of 0.01 mm between the tool 12 and work surfaces 10. The action of the servo 16 in micro-EDM is based on a measurement of the average gap-voltage between the tool 12 and the work 10. In micro-EDM, exceptionally low energy pulses with a small pulse duration are used to obtain the high accuracy required. Furthermore, discharge repetition rates are high, as over a million discharges are required to machine a hole of diameter xcx9c0.006 inches to a depth of xcx9c0.030 inches. The electrical pulses that are used to initiate discharges are much smaller; the objective is to have discharges of small energy, ideally of the order of 10xe2x88x927 to 10xe2x88x925 Joules, removing smaller increments of material from the work-piece 10.
To compensate for a possible fall in machining rate, because of lesser material removal, the frequency of the pulses is increased to a few orders of magnitude greater than die-sinking or wire EDM processes; for example, typical current pulse widths are 150 nanoseconds to 250 nanoseconds, and at rates of a million discharges a second. This causes the gap conditions to change rapidly. At such high discharge rates, the reliability of discharge repetition suffers with the use of oil-based dielectrics, conventionally used in EDM; to increase the reliability, de-ionized water is used as a dielectric 14.
In order to obtain holes of good quality with a smooth and damage-free surface, and to maintain consistency of dimensions from one hole to another, it is desirable that the sparking discharges occur in a controlled and uniform manner. While certain types of discharges produce surfaces with a good finish, other types of discharges are known to cause work surface damage or not remove material at all. In a typical machining cycle it is desirable that the fraction of xe2x80x9cgoodxe2x80x9d machining discharges be kept as high as possible. The state-of-the-art EDM machines for microhole drilling are not adequately equipped to discriminate between the various types of discharge pulses. They are only equipped with a servo 16 which controls the feed of the tool electrode 12 in such a way as to maintain a constant gap between the front faces of the electrodes 10, 12. Such servo systems 16 respond to the average voltage in the spark-gap which is not a sensitive indicator of the xe2x80x9cinstantaneousxe2x80x9d gap conditions or the efficiency of individual discharges (instantaneous gap condition meaning the condition existing in the machining gap during a single discharge pulse). Hence, the performance of the micro-EDM process is far from optimal. As a result, the process is not capable of fully meeting the exacting tolerance specifications required in many applications, such as for holes in fuel injectors for diesel engines and other similar applications in industry. Scrap levels are high, often as much as 30 percent and with the emission standards becoming more stringent (for diesel engines), there is a critical need for improving the micro-EDM process. In the diesinking application, previous researchers have characterized the instantaneous gap conditions in the EDM spark-gap through measurements of the gap voltage, gap current and radiation emissions in the radio-frequency band.
Several studies have been made of the nature of discharge pulses and their classification, based on measured electrical signals from the spark gap. For this discussion, the distribution of discharges occurring in EDM processes can be divided into four categories: (1) sparks; (2) arcs; (3) open circuits; and (4) short circuits. These four types of discharges have distinctive material removal properties. A spark discharge is characterized by the condition that when a voltage pulse is applied, the dielectric 14 breaks down, causing a steep reduction in the potential difference across the electrode and the work-piece. A pulse of current flows between the electrode 12 to work-piece 10 for a short duration, after which the discharge is quenched. If this discharge is characterized by high values for the voltage-time slopes and concomitant RF emissions that are high, it is classified as a spark discharge. Spark discharges have good material removal properties, in that a small amount of the work-piece 10 is eroded without much damage to the surrounding areas. After a discharge, there is a finite time needed for the dielectric 14 to de-ionize.
If subsequent voltage pulses are applied at the gap before this de-ionization is complete, then the potential difference between the tool 12 and work-piece 10 electrodes pulsates between zero and a fraction of the peak applied voltage. The voltage-time slope is lower, and the ensuing discharges, known as arcs, cause damage to portions of the work-piece 10 not machined. An arc is also characterized by a significantly lower RF emission than a spark. Arc-type discharges are more prevalent with the older RC type power supplies used in EDM. The term xe2x80x9cRCxe2x80x9d stands for Resistor and Capacitancexe2x80x94normally the charging and discharging of a capacitor through a resistor is a means for generating pulsed wave-forms, which are subsequently used in turning a power supply on and off. The product RC determines the frequency. It is noted that in many Micro-EDM applications, the percentage of such arcs is small, due to the use of the pulse-type power supply mechanism.
If the dielectric 14 strength is very low when a voltage pulse is applied, a short circuit results. In such a situation, there exists a continuous channel for current to flow between the electrode 12 and the work-piece 10. A short circuit can occur because of two reasons, namely, (a) the electrode being too close to portions of the work-piece that are not machined, or, (b) a situation where the removed material forms a debris close to the electrode in the machining zone. The second situation is often remedied by proper flowing of the dielectric medium, whereby the ejected material is removed at a regular rate. However, if the electrode is too deep inside the work-piece, more short circuits may occur due to the difficulty in removing the ejected material. It has been observed that this condition has a deleterious effect on machining, causing internal damage to the work-piece 10. A short circuit discharge train is characterized by almost no RF emission, high levels of current, and low values of the voltage-time slopes at discharge initiation.
In contrast to a short circuit, an open circuit refers to the absence of a discharge in the presence of a voltage pulse input. No material removal occurs during an open circuit condition, and consequently the duration of an open circuit should be kept within reasonable bounds. Thus, from the point of material removal, arcs and short circuits are bad discharges, while the spark type of discharge is desirable. The parameters of voltage slope and RF emission provide a means for discriminating between good and bad discharges.
Therefore, short circuits are undesirable disturbances that exist in the micro-EDM process. The lesser the short circuits in the process, the better the quality. Further, the machining time involved in the process can also be reduced if the percentage of xe2x80x9cnon-machiningxe2x80x9d discharges is reduced. The advantage of reduced machining time is in the reduction of the total cycle time, contributing to possible savings in capital investment. In micro-EDM, the total cycle time breaks down as follows: about 10 percent of the time is spent in preparation of the electrode, a process known as blunting; the loading and unloading of a part onto the machine takes about 10 percent of the cycle time; a flow test through the drilled components for dimensional tolerance takes about 5 percent, and the remaining 75 percent of the time is taken up in actual machining. Clearly, a significant reduction in this portion would contribute to a significant overall reduction in cycle time. For example, the drilling of micro-holes in fuel injector nozzles for atomization of fuel is done through the micro-EDM process. Despite possessing many advantages over conventional drilling for this application, a high cycle time is one of the major disadvantages of this process. A cycle time of 10 to 12 minutes for drilling eight holes in an injector nozzle is typical; a 12 minute hole typically includes 8 minutes for machining, 1 minute for electrode preparation, and 3 minutes for load, unload, and flow target checks through each nozzle. The total cycle time for six injectors per engine for this process alone is thus an hour and 12 minutes.
Maintaining a proper distance between the electrode 12 and the work-piece 10 is instrumental in determining the spark gap; this consequently determines the percentage of good spark discharges that occur between the electrode 12 and the work-piece 10. The electrode feed mechanism (servo 16) thus forms the principal component of an Electrical Discharge Machining (EDM) process; its function is to regulate the spark gap between the powered electrode 12 and the machined work-piece 10, whereby, (1) the formation of good machining discharges is effected; and (2) a fast recovery from bad machining conditions such as short circuits is ensured. Since sustained short circuits can cause a deterioration in metallurgical properties of the work-piece 10, an efficient feed and recovery mechanism must be designed to recover from or entirely prevent the occurrence of such conditions. A typical prior art feed mechanism is realized by coupling the electrode 12 to a servo-system 16. Typically, control schemes adopted in this servomechanism 16 are proportional, based on an averaged gap voltage feedback 20. Voltage signals applied to the electrode 12 in the EDM process are pulsed between zero and a maximum voltage at frequencies of several hundred kilohertz; since most actuators cannot respond at these frequencies, the gap voltage is filtered or averaged before being employed as a feedback signal 20. The motion of the actuator 16 is then regulated on the assumption that the actual gap between the electrode 12 is close to the work-piece 10 is proportional to the measured voltage difference; when the electrode 12 is close to the work-piece 10, the average potential difference between them falls to a percentage of the root mean squared value of the applied pulse voltage, and in the limiting case where both come into contact, the average potential difference is zero. Further, as the electrode 12 is drawn apart from the work-piece 10, the full applied voltage at the electrode 12 manifests as a potential difference with respect to the work-piece 10.
The traditional servomechanism 16 in EDM regulates the motion of the electrode 12 with a brush-less DC motor that is controlled by variations in the gap voltage through negative feedback 20. The electrode 12 is attached by means of a pneumatic clamp assembly to a mechanical slide. This slide is driven by the brush-less DC motor, with the rotation of the motor converted to linear motion of the slide by means of a lead screw or ball-screw. The typical stroke length employed in the drilling of fuel injector nozzles is about 1400xcexc to 2500xcexc, depending on the wall thickness of the nozzle. This is a unique property of injector nozzle holes, where the depth of the hole (wall thickness) is many times the diameter of the hole.
The stroke length and the short circuit response form the major components that decide the specification of an EDM servomechanism 16. With the typical time for a discharge in small hole drilling being about 150-250 nanoseconds, at rates of over a million discharges per second, the gap conditions change so rapidly that the servomechanism 16 needs to respond very quickly in order to maintain the correct discharge gap. Ideally, a frequency response of the order of megahertz would be required for an actuator 16 to maintain the correct discharge gap based on an instantaneous gap voltage feedback. At the very least, a frequency response of several kilohertz is required for the servomechanism 16 to respond accurately to changing conditions at the gap. While the servomechanisms 16 used for prior art EDM with the brush-less DC servo motors can achieve the stroke length requirement with ease, their frequency response with the inherent delays in the control systems is only about 200 Hz. This causes an inadequate response, or even instability in responding to short circuits. It is not uncommon to see the servo 16 not sensing a short circuit, and allowing the electrode 12 to feed further into the work-piece 12, thus accentuating the damage. When the servo 16 finally responds to a string of short circuits, it retracts too far behind, causing a string of open circuits in sequence. Note that during this time, no machining occurs. This duration thus causes a direct increase in machining cycle time.
Faster servomechanisms, on the other hand, do not possess a long stroke. For example, piezoelectric actuators have frequency responses on the order of several kilo-hertz, but the stroke length of actuators for such a high frequency response is limited to a maximum of 100xcexc. This is the typical conflict that occurs between the requirement of a long stroke and a high frequency response. It is very difficult for an actuator to satisfy both these requirements simultaneously; therefore, there remains a need in the industry for an actuator having an adequately long stroke and an adequately high frequency response. The present invention is directed toward meeting this need.
The present invention relates to an improved servomechanism for regulating the spark gap in micro electrical discharge machining (micro-EDM). The present invention utilizes a hybrid two actuator servo system for positioning the micro-EDM electrode. The hybrid system comprises a fast, easily controllable, short stroke actuator (such as a piezoelectric actuator) for good instantaneous response, and a second, slower actuator for positioning the fast actuator and for providing the required long stroke. This allows the slower actuator to xe2x80x9cfeedxe2x80x9d the electrode into the work-piece utilizing its long stroke, and the fast, short stroke actuator to respond quickly to instantaneous variations in the spark gap, such as short circuits.
In one form of the invention, a hybrid servomechanism for micro-electrical discharge machining is disclosed, comprising a first actuator having a first stroke length and a first frequency response; a platform coupled to said first actuator, wherein said first actuator is operative to move said platform; and a second actuator mounted to said platform and moving with said platform, said second actuator having a second stroke length and a second frequency response wherein the first stroke length is longer than the second stroke length and the first frequency response is slower than the second frequency response.
In another form of the invention, a hybrid servomechanism for micro-electrical discharge machining is disclosed, comprising a first actuator having a first stroke length and a first frequency response; a first slide coupled to said first actuator, wherein said first actuator is operative to move said slide; a base coupled to said first slide and moving with said first slide; a second slide in contact with said base, wherein said second slide may be moved independently of said base; a contact plate coupled to said second slide; a clamp coupled to said contact plate; and a second actuator having a first end coupled to said first slide and a second end coupled to said clamp to effect relative motion between said first slide and said second slide, wherein said second actuator has a second stroke length and a second frequency response wherein the first stroke length is longer than the second stroke length and the first frequency response is slower than the second frequency response.
In a further form of the invention, a method of controlling an electrode of a micro-electrical discharge machining device is disclosed, comprising the steps of (a) providing a first actuator having a first stroke length and a first frequency response; (b) providing a second actuator having a second stroke length and a second frequency response, wherein the first stroke length is longer than the second stroke length and the first frequency response is slower than the second frequency response; (c) feeding said electrode toward a work-piece using said first actuator; and (d) maintaining a predetermined spark gap between said electrode and said work-piece using said second actuator.